The Bushveld Complex, South Africa

Abstract

The mafic rocks of the Bushveld Complex, South Africa, were emplaced into a stable cratonic shield some 2.06 b.y. ago, and have remained remarkably well preserved from deformation, metamorphism and low temperature alteration, at least partly by its isostatic impact on the entire crustal thickness. The generation and emplacement of possibly 1 million km3 of magma within 65,000 years and the lateral continuity of layering (for up to 100 km) remain intriguing challenges to understanding the evolution of this igneous body. The intrusion is exposed as a three-lobed body, up to 7 km thick, with inward dipping layers that range from dunite to monzonite. Platinum-group element-rich orthopyroxenite, chromitite and vanadiferous magnetitite layers contain vast proportions of the World’s deposits of these commodities. Modal layering, on scales from mm to tens of m, ranges from well-developed in some vertical sections to virtually absent in others. Distinctive layers (ranging from mm to many tens of m) can be identified in two or even three lobes testifying to their connectivity. Feeders to the intrusion cannot be identified, and the exact compositions (and numbers) of the parental magmas are still debated. Rates and effectiveness of their mixing also require resolution. Models of magma additions and extents of mixing lead to very conflicting interpretations in terms of rapidity and vertical extents of the sequence affected. As the largest known mafic intrusion it represents an end-member in terms of magmatic chamber processes. I make the following variably contentious suggestions and conclusions. Many long vertical sections that show modal graded bedding (from orthopyroxenite to anorthosite) with no tendency for minerals to occur in their cotectic proportions may be best explained by grain settling and sorting due to gravity (applying Stokes’ Law). In contrast, in the chromitite and magnetitite layers marked vertical changes in composition have been attributed to in situ growth. Magma additions may be inferred from upward reversals in the mg# in olivine and pyroxene in the Lower and Main Zones, and upward increase in the An content of plagioclase in the Main Zone. Such reversals are not abrupt, but are preserved through thicknesses of 100 s of metres. Chromitite layers usually have sharp basal contacts and show no change in the mg# of the mafic mineral above, compared to below, each layer. Models of magma addition and rapid mixing appear inconsistent with such observations and mineral data. Orthopyroxenites in the Upper Critical Zone have an mg# < 82 that demonstrates that these minerals formed in equilibrium with plagioclase, but that plagioclase did not co-accumulate with pyroxene. Hence, these pyroxenites did not form from a magma saturated only in pyroxene. At the top of the Critical Zone there is Sr isotopic evidence for major addition of magma, but calculations using the Cr contents of pyroxene preclude any mixing between the magmas. The Cr content of magma needed for chromite saturation and the stability of olivine in the MELTS computer model do not produce crystallization sequences that match actual experimental observations on the MgO- and SiO2-rich liquids that have been proposed for the parental magmas to the Bushveld Complex As a consequence of the very slow accumulation rates of grains at the base of the chamber, combinations of Ostwald ripening and annealing caused consolidation into an essentially solid framework close to the grain-magma interface (no more than a few metres). Within most of the layered sequence, the small proportion of interstitial liquid that remained was effectively trapped. During its final solidification, this trapped liquid caused significant changes in mineral compositions, namely (i) decrease in mg# in pyroxenes where present in low abundance, and (ii) incompatible trace-element enrichment in all minerals relative to their original cumulus composition. These effects limit the ability to undertake geochemical modelling based on trace element abundances. Models that envisage introduction of grain slurries into their present locations either from below or from the sides create more problems than they solve, primarily in terms of producing distinctive layers of near-constant thickness over enormous areas, again by application of Stokes’ Law. Furthermore, the low Al2O3 content of orthopyroxene is inconsistent with their derivation from a deeper magma chamber. This chapter summarizes some of the main features of the Bushveld Complex, and then examines some of the main debates and challenges to understanding its magmatic history. Aspects of platinum-group element mineralization are reviewed in a companion chapter. Possibly we (I) err in trying to identify a single (or dominant) process, whereas many mechanisms may have been working in tandem with each being more effective in different situations. Many enigmas remain to be resolved, and there is little agreement regarding almost all aspects of the genesis of this huge body of rock.